The advent of the use of discrete multi-tone (DMT) modulation for digital subscriber line (DSL) modem technology applications, and its potential for mass deployment by local exchange carriers for high speed internet access, has presented telecommunication service providers with a unique challenge for testing digital subscriber (copper) loops. As diagrammatically illustrated in FIG. 1, a DSL modem 10 is intended to work over an existing copper pair 11 between a customer premises (CP) 12 and central office (CO) 14. Unlike traditional dial up modems, DSL modem signals do not pass through a PCM conversion process in the voice switch 16 of the central office equipment. Instead, the modem signal is demodulated at the central office and sent to a separate digital subscriber line access multiplexer (DSLAM) 17, which couples the digital data stream onto the network 18 for transmission to a remote site 19.
Although traditional dial-up modem performance is limited primarily by the PCM conversion process in the voice switch 16, rather than the characteristics of the copper loop 11, DSL modem performance and thus DSL service is directly dependent on the quality of the copper loop. Loop quality, in turn, is dependent on loop length, interference from sources within the cable plant and outside (radio stations, etc.), as well as bridge taps, load coils, splicing etc. It will be readily appreciated, therefore, that being able to sort cable pairs from an available bundle, based on their ability to provide a specific grade of service (data rate,) is essential to the economic deployment of DMT DSL.
As diagrammatically illustrated in FIG. 2, a DMT waveform provides multiple discrete tones (sub-carriers) 21. An illustrative example is Asymmetric Digital Subscriber Line (ADSL), having a 4.3125 KHz spacing between adjacent sub-carriers 21-i and 21-(i+1), over a spectrum from about 30 KHz to about 1.1 MHz. The effective or composite data rate of the ADSL DMT waveform is the sum of the data rate on each discrete sub-carrier. The data rate for each sub-carrier is ultimately governed by the signal-to-noise and distortion ratio (SINAD) for each 4.3125 KHz channel between 30 KHz and 1.1 MHz. Given an accurate estimate of SINAD for each sub-carrier channel, a maximum theoretical data rate or payload, that is independent of modem implementation, can be calculated for the loop.
One method to conduct loop testing would be to couple actual DMT modems to each end of the copper loop and perform a bit error rate (BER) measurement to grade the line. Unfortunately, BER measurements are directly dependent on the modem manufacture's receiver implementation. A different manufacturer may have more margin over some impairments and less over others. Thus, loop grading may be artificially skewed by the receiver implementation used in the conducting the BER test.